Of all modern European leaders, François Mitterrand was the one most clearly born into the wrong century. He would have made a superb Renaissance cardinal, presiding over mass with great pomp before retreating to a sumptuous apartment to engage in a little discreet selling of holy offices, before dinner with his mistress. He would have been a brilliant patron of the arts, a peerless schemer in the Curia, a deadly enemy even for a Borgia. A modern democracy was the wrong place for his talents. He succeeded in his greatest ambition, to rule France, but in the end he accomplished relatively little in the role.

He remains an endlessly fascinating figure, and Philip Short tells the story expertly in this deeply researched and marvellously readable biography. Born into a conservative, bourgeois family in south-western France in 1916, Mitterrand initially gravitated, like so many others in the feverish final days of the Third Republic, to the political extremes. He even moved in circles close to the rightwing terrorist group known as the Cagoule, without ever joining it himself. He also demonstrated a capacity for flamboyant passion by composing more than 2,000 love letters to the first great object of his affections, a young woman named Marie-Louise Terrasse. The war interrupted both his political and romantic plans, landing him in a German prisoner-of-war camp. But he soon escaped back to France, driven in large part by concerns about Marie-Louise's fidelity. And he then began a dangerous political dance, working overtly for the Vichy regime in prisoners' organisations while also flirting with the Resistance. Only in the summer of 1943 did he cast his lot with the latter, and demonstrated reckless bravery in the cause.

After the Liberation, his work on behalf of prisoners won him quick election to parliament, and a cabinet post when still just 30 years old. For nearly half a century thereafter he rarely disappeared from the public eye. He was anything but a populist, or a riveting orator. But he had astonishing personal magnetism. The journalist Françoise Giroud left a memorable description of Mitterrand's effect on women: "When he unwound, he was irresistible … If he had wanted to, he could have seduced a stone – economical in his gestures, his eyes shining with mischief, his voice velvety, his words enveloping you like a shawl." The same qualities helped him to forge powerful friendships with men as well, and to attract a large cadre of political supporters. It also helped that, like many other great politicians, he was a wholly convincing and absolutely shameless liar. As Georges Pompidou once remarked: "Never let Mitterrand impress you. No matter what he tells you, never believe a word he says."

His political odyssey was as vivid as his personality. During the decades after the war, most western European politicians moved steadily away from doctrinaire Marxian socialism; Mitterrand moved towards it. Already in 1940, his experience as a prisoner had shaken his confidence in traditional conservative politics. He was especially impressed by the way, in prisoner-of-war camps, an initial attitude of every man for himself gave way to co-operation. As he wrote, quite movingly, many years later: "One has to have seen the new representatives – nobody knew exactly how they had been appointed – dividing up the black bread into six slices, equal to the nearest millimetre, under the wide-eyed supervision of universal suffrage. It was a rare and instructive sight. I was watching the birth of the social contract." When he entered politics, it was as a member of the broad centre left, and he remained there throughout the 12 years of the Fourth Republic (1946‑58), during which he held ministerial posts on 11 separate occasions, and played an important role in the breakup of the French colonial empire.

When the Fourth Republic collapsed amid the debris of the Algerian war, and Charles De Gaulle took power, Mitterrand emerged as the most prominent leader of the opposition. He denounced De Gaulle for having staged a "permanent coup d'état" (even writing a book with this title), and ran unsuccessfully for president against him in 1965. He studied Marx and Lenin, and his rhetoric took on stronger Marxist accents. When a new Parti Socialiste emerged under his leadership in 1971, it had very different ideological goals from Britain's Labour, and from West Germany's Social Democrats. The next year, Mitterrand led it into an alliance with the Communist party, with their so-called Common Programme calling for large-scale nationalisations of industry.

During these years, Mitterrand's personal passions remained as strong as ever. After Marie-Louise jilted him, he fell into what can only be called a rebound marriage with Danielle Gouze, eight years his junior. She would bear him three children, but it was not exactly a marriage of minds. Soon after marrying, she innocently asked him how his day had gone. He snapped back: "I did not marry you under the regime of the Inquisition." He cheated on her incessantly, and with no apparent compunction. Then, in his late 40s, he met the 20-year-old Anne Pingeot, and fell for her as completely as he had done for Marie-Louise, although without entirely abandoning his other extracurricular activities. She became to all intents and purposes his second wife, bearing him a daughter, Mazarine, in 1974. Short's account, however, suggests that Danielle Mitterrand does not deserve too much of posterity's pity. In 1958 she acquired a long-term lover of her own, a gym teacher who sometimes fetched the morning croissants for the Mitterrand ménage, and then sat down to a friendly breakfast with François.

In many other countries, these escapades would have brought Mitterrand's career to a quick, scandalous conclusion. His relationship with Pingeot was widely known in Parisian society. But a compliant and complacent French press not only hid this secret until the end of Mitterrand's presidency in 1995, but two others as well. One, relatively inconsequential, was the youthful flirtation with the Cagoule. The other, much more serious, was that soon after taking office in 1981 Mitterrand was diagnosed with prostate cancer that had already metastasised to the bone. Remarkably, the secret did not leak out, and doctors kept him alive another 14 years.

The presidency itself saw Mitterrand's political journey to the left reach a sudden stop, and then reverse. His platform of large-scale nationalisation was not only anachronistic, but unworkable under conditions of global recession. Capital flight and social unrest ensued, and to stave off an economic meltdown, Mitterrand called for "opening a parenthesis in the history of socialism". It is a parenthesis that has never closed. He implemented austerity policies, sought a modus vivendi with French business and abandoned the full-bore Marxist rhetoric of the 1970s.

Mitterrand's 14 years in office (among post-Revolution French leaders, only Louis-Philippe and Napoleon III lasted longer) proved a moderate success in some respects. An ambitious decentralisation programme reduced the power of imperious Napoleonic prefects, and transferred considerable authority to newly created "regions" such as Rhône-Alpes and Aquitaine. Mitterrand barely bothered to disguise his strategy of "embracing" the Communist party in order to "smother" it, through an alliance that gave it little but four insignificant ministries. The arrangement hastened the communists' decline although their fall, in retrospect, looks entirely inevitable. In 1986, when Mitterrand lost control of the National Assembly to Jacques Chirac's neo-Gaullists, the resulting episode of "cohabitation" secured the Fifth Republic's stability, although the two men worked viciously to undermine each other. Mitterrand strengthened France's place in the western alliance, and worked effectively with Ronald Reagan, Margaret Thatcher and Helmut Kohl during the collapse of the Soviet bloc.

Short argues that Mitterrand "changed the ground rules of French social and political debate in ways more far-reaching and fundamental than any other modern leader before him". This is an exaggeration, for to a large extent, Mitterrand did little more than acquiesce in and preside over changes already under way. In fact, he arguably squandered the opportunity to move France towards a more open, flexible form of social democracy, loosening the dirigiste rigidity that still dominates so much of French economic planning and labour relations. He might, for instance, have done more to weaken the influence of the grandes écoles: the small, privileged institutions whose like-thinking graduates have a stranglehold on the highest circles of French government, business and academia. His weak gestures in this direction were summarily reversed by Chirac in 1986 and never reintroduced.

Mitterrand's failures came for two basic reasons. First, he had little understanding of, and less sympathy for, the way France had changed in the "30 glorious years" of postwar economic expansion. He had a sentimental fondness, born of his wartime experience, for industrial workers and peasants, but looked with distaste on the large new suburban middle class. A man of deep literary culture, and respect for tradition, he could not come to grips with men and women who preferred television to Stendhal, and fast food to cuisine bourgeoise. Short largely misses this side of Mitterrand. A skilled and fluid biographer, whose previous subjects include Mao and Pol Pot (Mitterrand must have come as something of a relief), he focuses on his subject's activities, often day by day, to the neglect of the larger historical context.

He does, however, capture well the other reason for Mitterrand's failures – a narcissism that was monstrous even by the standards of highly successful politicians. One of his anecdotes reveals it: at a crucial moment in the Algerian war, Mitterrand, then the minister responsible for French north Africa, kept a leading moderate Muslim politician waiting in his anteroom for an hour and a half because he insisted on catching up with the comics strips in the France Soir newspaper.

As president, once socialism's "parenthesis" had begun, Mitterrand lost interest in social transformation, and concentrated on manipulating those around him, in a manner that commentators compared, with reason, to the court of Louis XIV. Mitterrand himself was not entirely oblivious to this side of his personality. An earlier biographer, Catherine Nay, quoted his astonishing remark, made with tongue supremely in cheek, in reference to Stendhal's archetypical young man in a hurry: "Look at the poverty of Julien Sorel's ambition."

And, of course, Mitterrand also invested huge energies in the architectural grands projets that are one of his principal legacies: IM Pei's striking additions to the Louvre; the Grande Arche of La Défense; the Bastille Opera; the finance ministry at Bercy; the hideous National Library at Tolbiac that now bears Mitterrand's name. No other western leader of the last half-century did anything remotely comparable. It is a record a Renaissance potentate would have taken pride in.